The Monitoring of Cr(III) and Cr(VI) in Natural Water and Synthetic Solutions: An Assessment of the Performance of the Dgt and Dpc Methods

Abstract

The technique of diffusive gradients in thin films or diffusive gradient in thin films (DGT) has been used in this work for the in situ measurement of labile Cr(III) and Cr(VI) species. Direct measurement of Cr(VI) was also carried out in parallel with a field-based colourimetric technique based on the EPA 7196 diphenyl-carbohydrazide (DPC) method. The efficiency of the DGT and DPC methods were tested (a) in the laboratory, using synthetic solutions in the presence of realistic concentrations of Cr, humic substances (HS), and ethylenediaminetetraacetic acid (EDTA), and (b) in the field, in river water affected by effluents discharged by the tannery industry. The main advantage of the DGT method is that it allows the in situ separation of labile species of Cr(III) and Cr(VI), though there are still uncertainties about its performance in field conditions. The DPC method proved to be a fast, accurate, and relatively economical option for the field-based determination of Cr(VI). Sample acidification and ageing of unacidified samples from contaminated aquatic environments, produced significant errors in the determination of 'dissolved' Cr. The concentration of Cr(VI) determined by either the DGT or the DPC method exceeds recommended international guidelines.

Simultaneous speciation of chromium by spectrophotometry and multicomponent analysis

“…One of the most used colorimetric methods for the Cr(VI) andyor total chromium determination is based on the reaction of Cr(VI) with diphenylcarbazide (DPC) (Lynch et al, 1984;Andrade et al, 1985;Milacic et al, 1992;Sule et al, 1996;Clesceri et al, 2002;Mulaudzi et al, 2002;Giusti et al, 2005). The absorbance of the red -violet complex of unknown composition, formed at the pH range of 1.6 -2.2, is read at 540 nm (Clesceri et al, 2002).…”

Natural and anthropogenic contamination of the Fratta-Gorzone river (Veneto, Italy)

“…The parallel increase in 'dissolved' Cr load at the sites immediately downstream of the junction with the LEB canal can be explained with the flushing of dissolved Cr from sediment porewater and to the release of Cr associated with solid phases. Previous Cr speciation studies (Giusti and Barakat 2005) have shown an increase in the concentration of Cr(VI) along the very same river stretch, pointing to a possible change in redox conditions caused by the mixing of river water with oxygen-rich water from the LEB canal.…”

“…Given the chemical composition of the Fratta River and of the 'Tubo' discharge, it is also possible that precipitation of Cr species may take place. The saturation index (SI ¼ log 10 IAP K sp T ð Þ ) calculated (Giusti and Barakat 2005) with the hydrochemical model WATEQ4F (Ball and Nordstrom 2001) for the surface water investigated indicated supersaturation with respect to various Cr compounds. However, model calculations using the Windermere Humic Aqueous Model (WHAM 6.0) of Tipping (1998) indicated that small concentrations (1 mg/l) of humic substances would bind all the Cr in solution.…”

“…Total Cr in filtered stream water samples, blanks, and certified reference materials (surface water SRM 1943d, soft drinking water LGC 6011) was determined by ETAAS (electrothermal atomic absorption spectrometry) using a Unicam (Thermoelemental UK) with Zeeman correction and magnesium nitrate matrix modifier. Details of the ETAAS method, instrumental parameters, and furnace programme were published earlier (Giusti and Barakat 2005). The Cr detection limit was 0.015 ppb.…”

“…Downstream of site S14, the Fratta River changes name to Gorzone River and flows into the Brenta River. Water from the Adige River is pumped into the Fratta River at site L via the LEB canal canal, a study carried out in this catchment between 2000 and 2003 (Barakat 2004; Barakat and Giusti 2003a;Giusti and Barakat 2005) found that the load of pollutants in surface water was still responsible for high concentrations of sodium, chloride, chromium, and synthetic organic compounds. Also, a preliminary study (Barakat and Giusti 2003b) of bulk sediment samples collected along this river determined the association of Cr with operationally-defined phases (acid-soluble/exchangeable, reducible, oxidisable, and residual) using sequential leaching.…”